Coded aperture imaging has been proposed in the past as a means for improving the spatial resolution, sensitivity, and signal-to-noise ratio (SNR) of images formed by x-ray or gamma ray radiation. For many imaging applications, coded aperture cameras have proven advantageous relative to other candidate systems, including the single pinhole camera and multihole collimator systems. In contrast to these other systems, for instance, the coded aperture camera is characterized by high sensitivity, while simultaneously achieving exceptional spatial resolution in the reconstructed image.
In contrast to the single pinhole camera, coded aperture systems utilize multiple, specially-arranged pinholes to increase the overall photon transmission, and hence the sensitivity, of the imaging camera. In operation, radiation from the object to be imaged is projected through the coded aperture mask and onto a position-sensitive detector. The coded aperture mask contains a number of discrete, specially arranged elements that are either opaque or transparent to the incident photons. The raw signal from the detector does not reflect a directly recognizable image, but instead represents the signal from the object that has been modulated or encoded by the particular aperture pattern. This recorded signal can then be digitally or optically processed to extract a reconstructed image of the object.
Most commonly, coded aperture imaging techniques are employed in the field of astronomy. For instance, coded aperture techniques have been used for many years in the imaging of distant x- or gamma-ray astronomical sources. These applications are classic xe2x80x9cfar fieldxe2x80x9d (i.e. small object angle) imaging applications, where, under the proper conditions, conventional deconvolution techniques may be employed to produce ideal images of the source.
However, for near field applications, including nuclear medicine, molecular imaging, materials analysis, and contraband detection, among others, these same deconvolution methods do not produce ideal object images. Rather, in near field imaging applications, the reconstructed object images are corrupted by artifacts, including unsightly lines and/or bows through the object image. The presence of these artifacts render coded aperture systems far less attractive for near field imaging applications, particularly for applications such as nuclear medicine imaging, which rely heavily on visual inspection of the reconstructed object image. In the particular case of nuclear medicine imaging, for example, the presence of these artifacts may render it difficult or impossible to distinguish actual objects from artifacts in the reconstructed images, and may be a prohibitive factor weighing against the widespread adoption of coded aperture techniques.
The present invention relates to improved systems and methods for near-field coded aperture imaging of radiation-emitting sources. According to one aspect, the present invention is directed to reducing and/or eliminating the artifacts that are inherent in previous near field coded aperture imaging systems such that the improved sensitivity and resolution of these systems can be practically utilized.
In one aspect, the present invention is based upon the recognition that near field artifacts result primarily from the physical process of projecting radiation from the object through the coded aperture and onto the detector. It has heretofore been assumed that, for coded aperture arrays having perfect cross-correlation properties, the projected image obtained at the detector is the convolution of the object with the aperture mask. In fact, however, this projection contains a non-linear cos3(xcex8) term affecting the image. For far field applications, including astronomical applications where the object-to-detector distance is many orders of magnitude greater than the detector size, this cos3(xcex8) term can be approximated as equal to one, and the projected image on the detector is the straightforward convolution of the object and aperture mask.
For the near field applications of the present invention, however, this cos3(xcex8) term does not approximate as one, and therefore it gives rise to visible artifacts which corrupt the reconstructed object image. The present invention relates in one aspect to systems and methods for reducing or removing these artifacts from the reconstructed image. More specifically, the invention comprises providing a radiation-emitting object within the field of view of a coded aperture camera; generating a signal corresponding to a first image of radiation from the object projected through a first coded aperture mask pattern; generating a signal corresponding to a second image of radiation from the object projected through a second coded aperture mask pattern, where the second pattern is the xe2x80x9cnegativexe2x80x9d of the first; and then processing the combined data from these projections to obtain a reconstructed image of the object.
In the context of the present invention, the second mask pattern is related to the first mask pattern because the second pattern is the one associated to the negative of the decoding pattern associated with the first physical mask pattern. For each coded aperture mask pattern, there is an associated decoding function, G, that is used to decode the recorded image and produce the source reconstruction. One may change the sign of G to produce xe2x88x92G, i.e. the negative of the decoding array. The second projection is then taken with the physical mask associated with xe2x88x92G, which according to this invention is referred to as the xe2x80x9cnegativexe2x80x9d mask. As near-field artifact effects are dependent on G and xe2x88x92G, when the data sets from both projections are combined, the near-field artifacts cancel from the reconstructed image.
Some mask families, including the URA and m-sequence families, have the particular property that the xe2x80x9cnegativexe2x80x9d mask may be produced by simply interchanging the position of the opaque and transparent elements of the physical mask. This is because, for these families, the array, A, and decoding array, G, are identical. Thus, when signs change for the transparent and opaque elements of the decoding array, the elements simply interchange with one another. Also, because the array and decoding array are identical, the elements similarly change for the associated xe2x80x9cnegativexe2x80x9d physical mask. The resultant mask, with the elements interchanged, is sometimes referred to as the xe2x80x9cantimaskxe2x80x9d of the first mask.
For some near-field applications, two physical masks may be used to provide the two masks: the first being the mask pattern, and the second being the negative mask pattern. Depending on the particular mask family chosen, the second mask could be the antimask of the first. In this example, the physical masks can be interchanged between projections, either by operator intervention, or by an automated process.
In other cases, depending on the particular mask pattern used, a single physical mask may comprise both the mask and its negative mask. For some mask families, for instance, the negative of the original mask pattern (i.e. the mask associated with xe2x88x92G) may be obtained by simply rotating the physical mask by a given angle. For certain antisymmetric mask patterns, including a No-Two-Holes-Touching MURA pattern, for instance, a first mask pattern will produce its negative pattern when rotated by 90xc2x0.
The use of a mask and its antimask has been reported previously in connection with far-field astronomical applications for the much different goal of reducing non-uniform background and compensating for systematic irregularities. For instance, in the context of a balloon or satellite-borne gamma-ray telescope, the mask-antimask technique may be useful to xe2x80x9caverage outxe2x80x9d irregularities that may develop in the detector apparatus, particularly given the practical difficulties in accessing and actively maintaining the equipment.
On the other hand, near field coded aperture imaging applications uniquely suffer from the problem of near field artifacts, a problem which may be minimized through the use of a mask and its negative, which may, in certain instances, include the use of the mask/antimask pair. Generally, it has been found that when the data sets of an image taken through a mask and its negative mask are combined, the object images reinforce, while the near-field artifacts cancel. Thus, the mask/negative mask technique may be easily and efficiently employed in near field problems to produce robust images that are substantially free from artifacts caused by near-field geometry. The present invention is particularly advantageous in that it is implemented primarily in hardware, it does not rely on heavy computation and it does not require any significant extension of the time needed to produce an image.
In the context of the present invention, near-field imaging comprises coded aperture imaging applications wherein the projected image obtained at the detector is not the straightforward convolution of the object with the aperture mask, but is additionally affected by a cos3(xcex8) term that contributes to the reconstructed image in the form of visible artifacts. For many, but not necessarily all, near field applications, the object is less than about 10 meters from the detector. For other applications, including nuclear medicine imaging, where the goal is often to collect the data as close to the source as possible, the object-to-detector distance can be less than about 1 meter.
The present invention additionally relates to further improvements in the sensitivity and spatial resolution of coded aperture imaging applications. More particularly, the present invention relates to improvements in the design and fabrication of coded aperture masks for use in coded aperture imaging devices. Improvements can be made to the resolution, for instance, by selecting smaller pixel sizes for the opaque and transparent elements of the coded aperture mask. Also, it is possible to improve the signal-to-noise ratio for near-field applications by selecting an appropriate thickness for the mask. The signal-to-noise ratio may be further improved by selecting the appropriate mask pattern based on the particular characteristics of the near-field imaging problem at hand.